DC Compound Motors

• describe the torque, speed, rotation, and speed regulation and control
characteristics of a cumulative compound-wound dc motor.

• perform the preliminary test for the proper installation of a cumulative
compound motor.

• connect dc compound motors.

• describe the characteristics of a differential compound-wound dc motor.

• describe the characteristics of a cumulative compound-wound dc motor.

Compound-wound motors are used whenever it's necessary to obtain a speed
regulation and torque characteristic not obtainable with either a shunt
or a series motor. Since many drives need a fairly high starting torque and a constant speed under load, the com pound wound motor is suitable
for these applications. Some of the industrial applications include drives
for passenger and freight elevators, stamping presses, rolling mills, and metal shears.

The compound motor has a normal shunt winding and a series winding on
each field pole. As in the compound-wound dc generator, the series and shunt
windings may be connected in long shunt, or short shunt.

ill. 1 Motor field connections: (A) Long Shunt; (B) Short Shunt

When the series winding is connected to aid the shunt winding, the machine
is a cumulative compound motor. When the series field opposes the shunt
field, the machine is a differential compound motor. Using Fleming’s right-hand
rule for electromagnets, it can be seen that the two windings will either
reinforce each other or try to ca each other.

ill. 2 Compound field windings: SERIES WINDING, GENERATOR FIELD
POLE

TORQUE

The operating characteristics of a cumulative compound-wound motor are
a combi nation of those of the series motor and the shunt motor. When a
load is applied, the increasing current through the series winding increases
the field flux. As a result, the torque for a given current is greater
than it would be for a shunt motor. However, this flux increase causes
the speed to decrease to a lower value than in a shunt motor. A cumulative
compound-wound motor develops a high torque with any sudden increase of
load. It is best suited for operating varying load machines such as punch
presses.

SPEED

Unlike a series motor, the cumulative compound motor has a definite no-load
speed and won't build up to destructive speeds if the load is removed.

Speed Control

The speed of a cumulative compound motor can be controlled by the use
of resistors in the armature circuit to reduce the applied voltage. When
the motor is to be used for installations where the rotation must be reversed
frequently, such as in elevators, hoists, and railways, the controller
used should have voltage dropping resistors and switching arrangements
to accomplish reversal.

Electronic Speed Control

A block diagram approach to the electronic speed control of a dc motor
is presented in figure 3. The ac line is rectified by a full wave bridge
to supply pulsating dc to the shunt field at a steady value. The dc supplied
to the armature and the series field is con trolled by an SCR (silicon
controlled rectifier). By adjusting the firing time of the SCR, either
more or less of the DC voltage available can be applied to the armature.
If a low amount of dc voltage is applied, the torque is low and the resultant
speed is low. If the SCR is fired early in the waveform and allowed to
conduct for most of the cycle, then a larger amount of voltage is applied
to the armature, more current results, and torque increases to spin the
armature at a higher speed. Speeds below rated motor speed are produced
by lowering the amount of voltage applied to the armature while keeping
the shunt field steady; if higher-than-normal speed is needed, the shunt
field can be weakened. Most speed controls have some type of feedback to
sense the armature current, compare it to the set speed, and adjust the
SCR firing angle to compensate for varying mechanical loads to keep the
motor speed regulated.

The rotation of a compound-wound motor can be reversed by changing the
direction of the current in the field or the armature circuit, ( 5). Since the series field coils must also be reversed if the shunt field
is reversed, it's easier to reverse the current in the armature only.

When a motor is first connected, it's important to determine the continuity
of the shunt field circuit. In addition, for a compound-wound motor, the
proper magnetic polarity of the shunt and series field must be determined.
Standardized tests determine these conditions. E.g., when the motor
is connected to the controller and is ready for starting, disconnect the
armature wire at the motor, close the line switch, and place the starter
on the first contact point. Open the line switch slowly. If the field is
intact there will be an arc at the switch. The absence of a spark indicates
an open field circuit. This fault must be located and corrected before
proceeding. A motor ordinarily won't start on an open field, but if
it does start, it will race.

When the shunt field circuit tests complete, the motor should be started
as a shunt motor. If the motor operates satisfactorily in the desired direction
of rotation, disconnect the motor. If it rotates in the opposite direction,
disconnect it and reverse the shunt field leads. Restart the motor. If
it now rotates in the desired direction, mark the leads and disconnect
power.

Next, open the shunt field circuit, connect in the series field, and operate
the motor momentarily as a series machine. As soon as the armature begins
to turn, note the direction of rotation and disconnect the motor power.

If the armature rotates in the desired direction, connect in the shunt
field circuit and the motor is ready for operation. If the direction of
rotation as a series motor is opposite to the desired direction, reverse
the series field leads and then connect the shunt field circuit. The motor
is now ready for operation. This makes sure the shunt field and series
field help each other.

Differential Compounding

Excellent speed regulation can be obtained with a differential compound
motor. When a motor is connected as a differential compound machine, the
series field opposes the shunt field so that the field flux is decreased
as a load is applied ( 6). As a result, the speed remains substantially
constant with an increase in load. With over-compounding, a slight increase
in speed is possible with an increase in load. This speed characteristic
is achieved only with a loss in the rate at which torque increases with
load.

ill. 6 Magnetic polarities of compound motors:

DIFFERENTIAL:

• CONSTANT SPEED

• LOW STARTING TORQUE

CUMULATIVE:

• SERIES CHARACTERISTICS

• HIGH STARTING TORQUE

Since the field decreases with a load increase, a differential compound
motor has a tendency to speed instability. When starting a differential
motor, it's recommended that the series field be shorted since the great
starting current in this field may overbalance the shunt field and cause
the motor to start in the opposite direction.

A differential machine is connected and tested on installation, using
the same procedure outlined for a cumulative compound motor. For the differential
motor, however, the series windings should be connected in the opposite
direction from that of the shunt winding. Don't exceed load on nameplate
or reversal of direction may occur.

SUMMARY:

The dc compound motor is used where a compromise is needed between the
series and the shunt motor. The compound motor has better speed characteristic
than the series motor and better torque characteristics than the shunt
motor. The motor can be connected so that the shunt field and the series
fields are in the same direction. This is the cumulative connection. You
should perform a test to be sure the motor is connected as intended. The
cumulative connection reacts quite differently than the differential connection.
Many dc motors are now driven by electronic drives. The same basic concepts
are used to control speed and direction.

QUIZ:

1. Circle the letter for each of the following statements which applies
to a cumulative compound-wound dc motor.

a. The speed regulation of a cumulative compound-wound dc motor is better
than that of a shunt motor.

b. The speed of a cumulative motor has a no-load limit.

c. The speed of the motor decreases more for a given increase in load
than does a differential motor.

d. A cumulative motor has less torque than a shunt motor of the same hp
rating for a given increase in armature current.

e. The speed regulation of a cumulative motor is better than that of a
series motor.

f. A cumulative motor develops a high torque with a sudden increase of
load.

g. To reverse the direction of rotation, the current in either the armature
or the shunt field must be reversed.

h. A cumulative motor is connected so that the series flux aids the shunt
winding flux.

i. When installing a cumulative compound-wound motor, the direction of
rotation should be the same when testing the motor for operation either
as a series motor or a shunt motor.

2. Circle the letter for each of the following statements which applies
to a differential compound-wound dc motor.

a. A differential motor is used in applications where an essentially constant
speed at various loads is required.

b. The starting torque for a differential motor is higher than that of
a cumulative motor.

c. The motor may reverse its direction of rotation if started under a
heavy load.

d. This motor develops a speed instability since the flux field decreases
with a load increase.

e. When starting a differential motor, the shunt field should be shorted
because of the great starting current.

3. Arrange the following steps numerically in the correct sequence to
test for the proper connections to operate a cumulative compound-wound
motor. Place the step number in the space provided, starting with number
1.

____a. Place starter on first contact point to test field.

____b. With motor shut down, open shunt field circuit.

____c. If rotation is in direction opposite to that desired, reverse the
series field leads.

____d. Disconnect armature wire at motor and close line switch.

____e. An absence of spark indicates an open field circuit which must
be located and corrected before proceeding with the test.

____f. If rotation is in direction opposite to that desired, shut down
motor, reverse shunt field leads, and restart motor; rotation should be
in desired direction.

____g. Slowly open line switch and observe for arc at switch indicating
field is intact.

____h. Start motor as a shunt motor and observe rotation.

____i. Connect in series field, start motor and immediately shut it down
while noting direction of rotation.

____j. Connect the shunt field circuit; the motor is now ready for operation.